This study is to see if a mix of medicines can help kids with tough-to-treat tumors. **Sirolimus** is a medicine taken by mouth once a day, and **celecoxib** is another medicine that helps reduce swelling. They are given with two other medicines, **etoposide** and **cyclophosphamide**, which are taken every 21 days. This mix is called metronomic chemotherapy, which means giving small, regular doses of drugs.

The goal is to find out if this combination can shrink tumors and how long it can stop them from growing. The study also wants to learn about side effects and how the treatment affects the blood and tumor tissue. Kids with certain types of relapsed or resistant tumors can join, except those with some specific brain tumors.

• The study involves regular health checks and tests.
• Participants should not be pregnant or breastfeeding.
• There are specific requirements for blood and organ function.

Kids need to meet certain health conditions to join, and there are restrictions like not using cannabis oil in the beginning. If you are interested, more details can be found in the study protocol.